Fossil finds shake up dinosaur theories

Dinosaur fossils found in New Mexico are challenging the idea that when dinosaurs appeared on the scene some 235 million years ago, they quickly rose to dominate the landscape.

Buried among the dinosaur bones, a team led by UC Berkeley paleontologists discovered the remnants of the dinosaurs' predecessors, dinosauromorphs, that lived 15-20 million years after the first dinosaur showed up.

"It was very exciting because we knew this was a type of animal that no one thought you'd find anywhere at any time in North America," said paleontologist Randall Irmis, a graduate student at UC Berkeley and lead author of the study which appears today in Science.

The discovery means that dinosaurs didn't simply replace their ancestors. Instead, the two types of animals lived side-by-side and competed for resources for millions of years.

"It has shaken up the old theory," said Bill Parker, a paleontologist at Petrified Forest National Park who also studies dinosaurs. "Everything was nice and neat before."

Scientists thought dinosaurs evolved from the dinosauromorphs in South America. Then, they may have driven their predecessors to extinction by outcompeting them with their bigger, faster and stronger bodies. Or, their ancestors and other animals suddenly went extinct for another reason, and the dinosaurs took advantage of the newly empty ecological niches.

Either way, the belief was that by the time dinosaurs were roaming North America, the dinosauromorphs were long gone.

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"Everybody thought those animals had gone extinct," Parker said. "I think people are going to be surprised."

Previously, there was little or no overlap in time between dinosaur fossils, which first appear around 235 million years ago, and dinosauromorphs which seemed to disappear around the same time, until a single younger dinosauromorph was found in Poland in 2003.

The New Mexico find is the first with dinosaurs and their precursors living in the same place at the same time, and the bones are even younger than the Polish fossils.

Paleontologist Paul Sereno of the University of Chicago isn't convinced. "I think the more gradual picture of dinosaurs painted by the authors is more speculative," he said. "In South America, where we have the only dated sequence of early fuanas leading to dinosaur dominance, it seems to occur fairly quickly, just a few million years."

Sereno agrees that the New Mexico find shows that in some places, dinosauromorphs lived on alongside the dinosaurs. But he contends the dinosaurs did quickly replace the most important animals that were dominant in numbers, the mammallike reptiles, except for a few stragglers that were closer to crocodiles than dinosaurs.

Irmis and his team had been interested in the New Mexico quarry simply because it contained early dinosaurs between 210 and 220 million years old, which are rare in North America. But then they came across some unexpected femurs and other bones of the same age that resembled a type of dinosauromorph found in Argentina.

Irmis went to South America to compare the fossils and confirmed they were related. They named the new North American species Dromomeron romeri from the Greek words dromas and meros meaning running femur, and Alfred Sherwood Romer, the scientist who first described the Argentine species.

Dromomeron was probably between three and five feet long and may have walked on two feet. The team also found bones from another dinosauromorph that was a beaked herbivore that walked on four feet and was three times the size of Dromomeron.

Since the discovery, the paleontologists have found more Dromomeron bones that had been overlooked by other scientists at other known dinosaur fossil locations in New Mexico, Arizona and Texas.

"They just didn't have a search image in their mind to keep a look out for these things because they weren't expecting them to be there," Irmis said.

The coexistence of dinosaurs with their ancestors shows how evolution really works, Parker said.

"People think evolution is supposed to be a straight line from one form of animal to another," he said. " But it's actually more like a branching tree and this is a prime example of that."

Betsy Mason covers science and the national laboratories. Reach her at 925-847-2158 or bmason@cctimes.com.